SPINDEX: Measure Equivalent Widths at each Row of an Image

SPINDEX measures line-strengths indices in a 1-D or a 2-D (long-slit, multi-object) image. The input file must contain a list of the wavelength intervals needed to measure each index.

As an example, here is a file that would measure some of the spectral indices used by S.M. Faber (UCO/Lick Observatory) and J.J. Gonzalez (UNAM) for elliptical galaxies or spiral bulges (the Lick Spectral Indices):

The Faber & Gonzalez Lick Spectral Indices
I Index band   blue continuum   red continuum   Units Name
01 4847.875 4876.625 4827.875 4847.875 4876.625 4891.625 0 H-beta
02 4903.000 4945.500 4894.500 4907.000 4943.750 4954.500 0 Fe4930
03 4977.750 5054.000 4946.500 4977.750 5054.000 5065.250 0 Fe5015
04 5245.650 5285.650 5233.150 5248.150 5285.650 5318.150 0 Fe5270
05 5312.125 5352.125 5304.625 5315.875 5353.375 5363.375 0 Fe5335
06 5387.500 5415.000 5376.250 5387.500 5415.000 5425.000 0 Fe5406
07 5160.125 5192.625 5142.625 5161.375 5191.375 5206.375 0 Mgb
08 5069.125 5134.125 4895.125 4957.625 5301.125 5366.125 1 Mg1
09 5154.125 5196.625 4895.125 4957.625 5301.125 5366.125 1 Mg2
  starts ends starts ends starts ends 0=Ang Name

Comment lines are ignored when the file is read and can be placed anywhere. The relevant information rest in the first 8 columns of each index entry and the order has to be in the example. The continuum intervals are used to define a linear pseudo-continuum level. The actual measurement is carried integrating the intensity-to-continuum ratio over the Index wavelength-interval. If the units flag is set to 0, the Index will be measure in angstroms as an equivalent width. If the units flags is not zero, the Index will be a measure (in magnitudes) of the intensity-to-continuum ratio :

  Index = integral(1-I/C) over the Index Band            (zero units flag)

  Index = -2.5 log (integral(I/C) over the Index Band)   (non-zero units flag)

All wavelength scales (linear, polynomial or logarithmic wavelength, or linear in any other units) are accepted. In case of units other than angstroms, the list in the input file has to be scaled accordingly.

Flexure shifts (in pixels and/or angstroms), velocity shifts (in km/sec or redshift), and shifts due to rotation or line-curvature (for long-slit or multi-object spectroscopy), can be taken into account in placing the intervals on the spectrum with keywords DP=, DLAM=, V=, Z=, and/or ROT=.

Some users carry along their reduction an image with an estimation of the noise-variance at each pixel. SPINDEX can use this image (VAR=buf keyword) to estimate an error for the index. It is important to know that the intensity values of the variance buffer are not used as weights for the integration of the index, but noise-variances used to estimate the error on the index. These are not sigmas, but variances (more like sigma-squared) because as must users know, error-propagation in data reduction is simpler dealing with variances.

The output buffer will have as many rows as the source image and as many columns as the number of indices. So dest(r,c) will be the value of the c-th index at row r. In case the error where estimated with the VAR=buf keyword, the output image will have twice plus one the number of columns (indices), saving indices and errors in the format used by command ROWFIT to save fit parameters. In this case dest(r,c) will have the c-th index of row r, and dest(r,-c) its error.

The results can be printed with the TTY keyword or using command 'PRINT dest', both accept output redirection to an ASCII file.